It would be useful to have a sensor to measure the density of a gas or a mixture of gases in chemical and physical processes. Density is normally not measured for gased because there are no simple methods to sense density directly. Optical interferometry can be used but this is not simple for process control applications. One technique is to measure the acoustic wave transmitted in an enclosure as proposed by Haran, Tech. Mess. 50, 43 (1983). In Haran's analysis, the pressure at the receiver located opposite a transmitter driven at a fixed velocity amplitude, is proportional to the gas density only. However, the measurements of eight different gases did not show linearity with gas density. The assumption by Haran that there exist plane waves with pressure nodes at x=0, i.e. transmitter, a requirement for linearity in density, we believe is faulty.
More recently, an acoustic method to measure gas density has been proposed in which sound produced by a transducer is detected a small distance away by another transducer; see Haran, Rev. Sci. Instrum. 59, 2059 (1988). Under proper conditions, this concept can be used to measure on-line gas density. An analysis of such proper conditions are discussed below in which our analysis shows that dipole-type sources in small enclosures, operated at low frequencies, can provide low-cost, effective, density measurement cells. We also show that a small tube driven by sources at the closed ends in opposite phases with a microphone located a short distance sway from one source also produces a density measurement cell of convenient design.